There is a growing demand for high purity metals, particularly high purity silicon of solar grade and electronic grade. Solar grade silicon feedstock for solar cells has been based on scrap/rejects from electronic grade silicon from the semiconductor industry. Electronic grade silicon is produced by production of silane from metallurgical silicon and gas phase reduction of silane to silicon. This process is, however, very costly. In addition the availability of scrap/rejects from the semiconductor industry is now too small to supply the fast growing solar cell market.
From U.S. Pat. No. 3,219,561 it is known a method for producing refined silicon and germanium by passing a direct current between an anode in contact with a molten salt bath containing a fluoride and an oxide of silicon or germanium and a cathode in contact with another molten salt bath containing of fluoride where the baths are separated by a molten alloy of silicon or germanium and another metal to reduce the oxide of silicon or germanium to silicon or germanium and deposit it on the cathode. In this electrolytic process the silicon or germanium are deposited as a solid on the cathode. The solid metal has to be removed from the cathode and has to be crushed and treated by acids in order to remove impurities trapped in the metal deposited on the cathode.
In U.S. Pat. No. 3,254,010 there is disclosed another method for refining impure silicon or germanium where a current is passed between a cathode and an anode through a molten salt electrolyte containing a fluoride, where the anode is made from impure silicon or germanium or alloys of impure silicon or germanium with more noble metals than silicon or germanium to deport on the cathode refined silicon or germanium. Also in this process solid refined silicon or solid refined germanium are deposited on the cathode. The electrolyte is preferably cryolite. The process of U.S. Pat. No. 3,254,010 thus has the same drawbacks as the method of U.S. Pat. No. 3,219,561.
Finally, for metals having relatively low melting points such as aluminium, electrolytic refining is a conventional process and is described in U.S. Pat. No. 1,534,318. In this patent it is described a process for electrolytic refining of aluminium where there is established a lower layer of molten metal containing aluminium as an anode, an upper layer or of molten aluminium as cathode and an intermediate layer of molten electrolyte of a greater density than the molten aluminium, which electrolyte is essentially fluorides and substantially free from chloride. Current is passed from the anode metal through the electrolyte to the aluminium cathode where aluminium is removed from the anode metal and deposited in the molten state at the cathode. The molten electrolyte contains aluminium and sodium fluorides and between 20 and 60% of fluoride of an alkali earth metal having an atomic weight greater than 80, for example barium fluoride.
The above process disclosed in U.S. Pat. No. 1,534,318 can, however, not be used for electrolytic refining of metals having a high melting point of above 1000° C. since a lot of fluoride vapour would form in the fluoride-based molten electrolyte at such high temperatures destroying the properties of the electrolyte.
There is therefore a need for a process whereby high purity, high melting metals such as silicon can be refined by an electrolytic refining process.